PDH-Pro.com 396 Washington Street, Suite 159, Wellesley, MA 02481 Telephone – (508) 298-4787 www.PDH-Pro.com This document is the course text. You may review this material at your leisure before or after you purchase the course. In order to obtain credit for this course, complete the following steps: 1) Log in to My Account and purchase the course. If you don’t have an account, go to New User to create an account. 2) After the course has been purchased, complete the quiz at your convenience. 3) A Certificate of Completion is available once you pass the exam (70% or greater). If a passing grade is not obtained, you may take the quiz as many times as necessary until a passing grade is obtained (up to one year from the purchase date). If you have any questions or technical difficulties, please call (508) 298-4787 or email us at [email protected]. Subsurface Drilling and Sampling Course Number: GE-02-601 PDH: 4 Approved for: AK, AL, AR, GA, IA, IL, IN, KS, KY, MD, ME, MI, MN, MO, MS, MT, NC, ND, NE, NH, NJ, NM, NV, OH, OK, OR, PA, SC, SD, TN, TX, UT, VA, WI, WV, and WY New Jersey Professional Competency Approval #24GP00025600 North Carolina Approved Sponsor #S-0695
Transcript
PDH-Pro.com
396 Washington Street, Suite 159, Wellesley, MA 02481 Telephone – (508) 298-4787 www.PDH-Pro.com
This document is the course text. You may review this material at your leisure before or after you purchase the course. In order to obtain credit for this course, complete the following steps: 1) Log in to My Account and purchase the course. If you don’t have an account, go to New User to create an account. 2) After the course has been purchased, complete the quiz at your convenience. 3) A Certificate of Completion is available once you pass the exam (70% or greater). If a passing grade is not obtained, you may take the quiz as many times as necessary until a passing grade is obtained (up to one year from the purchase date). If you have any questions or technical difficulties, please call (508) 298-4787 or email us at [email protected].
Subsurface Drilling and Sampling
Course Number: GE-02-601
PDH: 4
Approved for: AK, AL, AR, GA, IA, IL, IN, KS, KY, MD, ME, MI, MN, MO, MS, MT, NC, ND, NE, NH, NJ, NM, NV, OH, OK, OR, PA, SC, SD, TN, TX, UT, VA, WI, WV, and WY
New Jersey Professional Competency Approval #24GP00025600 North Carolina Approved Sponsor #S-0695
U.S. Department Publication No. FHWA NHI-01-031of Transportation May 2002Federal HighwayAdministration
NHI Course No. 132031
Subsurface Investigations— Geotechnical Site Characterization
Reference Manual
National Highway Institute
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CHAPTER 3.0
DRILLING AND SAMPLING OF SOIL AND ROCK
This chapter describes the equipment and procedures commonly used for the drilling and sampling of soil androck. The methods addressed in this chapter are used to retrieve soil samples and rock cores for visualexamination and laboratory testing. Chapter 5 discusses in-situ testing methods which should be includedin subsurface investigation programs and performed in conjunction with conventional drilling and samplingoperations.
3.1 SOIL EXPLORATION
3.1.1 Soil Drilling
A wide variety of equipment is available for performing borings and obtaining soil samples. The method usedto advance the boring should be compatible with the soil and groundwater conditions to assure that soilsamples of suitable quality are obtained. Particular care should be exercised to properly remove all sloughor loose soil from the boring before sampling. Below the groundwater level, drilling fluids are often neededto stabilize the sidewalls and bottom of the boring in soft clays or cohesionless soils . Without stabilization,the bottom of the boring may heave or the sidewalls may contract, either disturbing the soil prior to samplingor preventing the sampler from reaching the bottom of the boring. In most geotechnical explorations, boringsare usually advanced with solid stem continuous flight, hollow-stem augers, or rotary wash boring methods.
Solid Stem Continuous Flight Augers
Solid stem continuous flight auger drilling is generally limited to stiff cohesive soils where the boring wallsare stable for the entire depth of the boring. Figure 3-1a shows continuous flight augers being used with adrill rig. A drill bit is attached to the leading section of flight to cut the soil. The flights act as a screwconveyor, bringing cuttings to the top of the hole. As the auger drills into the earth, additional auger sectionsare added until the required depth is reached.
Due to their limited application, continuous flight augers are generally not suitable for use in investigationsrequiring soil sampling. When used, careful observation of the resistance to penetration and the vibrationsor "chatter" of the drilling bit can provide valuable data for interpretation of the subsurface conditions. Clay,or "fishtail", drill bits are commonly used in stiff clay formations (Figure 3-1b). Carbide-tipped "finger" bitsare commonly used where hard clay formations or interbedded rock or cemented layers are encountered.Since finger bits commonly leave a much larger amount of loose soil, called slough, at the bottom of the hole,they should only be used when necessary. Solid stem drill rods are available in many sizes ranging in outsidediameter from 102 mm (4.0 in) to 305 mm (12.0 in) (Figure 3-1c), with the 102 mm (4.0 in) diameter beingthe most common. The lead assembly in which the drill bit is connected to the lead auger flight using cotterpins is shown in Figure 3-1d. It is often desirable to twist the continuous-flight augers into the ground withrapid advancement and to withdraw the augers without rotation, often termed “dead-stick withdrawal”, tomaintain the cuttings on the auger flights with minimum mixing. This drilling method aids visualidentification of changes in the soil formations. In all instances, the cuttings and the reaction of the drillingequipment should be regularly monitored to identify stratification changes between sample locations.
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(a) (b)
(c) (d)
Figure 3-1. Solid Stem Continuous Flight Auger Drilling System: (a) In use on drill rig, (b) Finger andfishtail bits, (c) Sizes of solid stem auger flights, (d) Different assemblies of bits and auger flights. (Allpictures in the above format are courtesy of DeJong and Boulanger, 2000)
Hollow Stem Continuous Flight Augers
In general hollow stem augers are very similar to the continuous flight auger except, as the name suggests,it has a large hollow center. This is visually evident in Figure 3-3a, where a solid stem flight and a hollowstem flight are pictured side-by-side. The various components of the hollow stem auger system are shownschematically in Figure 3-2 and pictured in Figure 3-3b to 3-3f. Table 3-1 presents dimensions of hollow-stem augers available on the market, some of which are pictured in Figure 3-3c. When the hole is beingadvanced, a center stem and plug are inserted into the hollow center of the auger. The center plug with a dragbit attached and located in the face of the cutter head aids in the advancement of the hole and also preventssoil cuttings from entering the hollow-stem auger. The center stem consists of rods that connect at the bottomof the plug or bit insert and at the top to a drive adapter to ensure that the center stem and bit rotate with theaugers. Some drillers prefer to advance the boring without the center plug, allowing a natural "plug" ofcompacted cuttings to form. This practice should not be used since the extent of this plug is difficult tocontrol and determine.
Once the augers have advanced the hole to the desired sample depth, thestem and plug are removed. A sampler may then be lowered through thehollow stem to sample the soil at the bottom of the hole. If the augers havebeen seated into rock, then a standard core barrel can be used.
Hollow-stem augering methods are commonly used in clay soils or ingranular soils above the groundwater level, where the boring walls may beunstable. The augers form a temporary casing to allow sampling of the"undisturbed soil" below the bit. The cuttings produced from this drillingmethod are mixed as they move up the auger flights and therefore are oflimited use for visual observation purposes. At greater depths there maybe considerable differences between the soil being augered at the bottomof the boring and the cuttings appearing at the ground surface. The fieldsupervisor must be aware of these limitations in identification of soilconditions between sample locations.
Significant problems can occur where hollow-stem augers are used tosample soils below the groundwater level. The hydrostatic water pressureacting against the soil at the bottom of the boring can significantly disturbthe soil, particularly in granular soils or soft clays. Often the soils will heave and plug the auger, preventingthe sampler from reaching the bottom of the boring. Where heave or disturbance occurs, the penetrationresistance to the driven sampler can be significantly reduced. When this condition exists, it is advisable tohalt the use of hollow-stem augers at the groundwater level and to convert to rotary wash boring methods.Alternatively the hollow-stem auger can be flooded with water or drilling fluid to balance the head; however,this approach is less desirable due to difficulties in maintaining an adequate head of water.
Note: Adapted after Central Mine Equipment Company. For updates, see: http://www.cmeco.com/
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(d)
(a)
(c)
(e) (f)
(b)
Figure 3-3. Hollow Stem Continuous Flight Auger Drilling Systems: (a) Comparison with solidstem auger; (b) Typical drilling configuration; (c) Sizes of hollow stem auger flights;(d) Stepwise center bit; (e) Outer bits; (f) Outer and inner assembly.
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Figure 3-4. Schematic of Drilling Rig for Rotary Wash Methods(After Hvorslev, 1948).
Rotary Wash BoringsThe rotary wash boring method (Figures 3-4 and 3-5) is generally the most appropriate method for use in soilformations below the groundwater level. In rotary wash borings, the sides of the borehole are supportedeither with casing or with the use of a drilling fluid. Where drill casing is used, the boring or is advancedsequentially by: (a) driving the casing to the desired sample depth,(b) cleaning out the hole to the bottom ofthe casing, and (c) inserting the sampling device and obtaining the sample from below the bottom of thecasing.
The casing (Figure 3-5b) is usually selected based on the outside diameter of the sampling or coring tools tobe advanced through the casing, but may also be influenced by other factors such as stiffness considerationsfor borings in water bodies or very soft soils, or dimensions of the casing couplings. Casing for rotary washborings is typically furnished with inside diameters ranging from 60 mm (2.374 in) to 130 mm (5.125 in).Even with the use of casing, care must be taken when drilling below the groundwater table to maintain a headof water within the casing above the groundwater level. Particular attention must be given to adding waterto the hole as the drill rods are removed after cleaning out the hole prior to sampling. Failure to maintain anadequate head of water may result in loosening or heaving (blow-up) of the soil to be sampled beneath thecasing. Tables 3-2 and 3-3 present data on available drill rods and casings, respectively.
For holes drilled using drilling fluids tostabilize the borehole walls, casingshould still be used at the top of the holeto protect against sloughing of the grounddue to surface activity, and to facilitatecirculation of the drilling fluid. Inaddition to stabilizing the borehole walls,the drilling fluid (water, bentonite, foam,Revert or other synthetic drillingproducts) also removes the drill cuttingsfrom the boring. In granular soils andsoft cohesive soils, bentonite or polymeradditives are typically used to increasethe weight of the drill fluid and therebyminimize stress reduction in the soil atthe bottom of the boring. For boringsadvanced with the use of drilling fluids, itis important to maintain the level of thedrilling fluid at or above the groundsurface to maintain a positive pressurefor the full depth of the boring.
Two types of bits are often used with therotary wash method (Figure 3-5c). Dragbits are commonly used in clays andloose sands, whereas roller bits are usedto penetrate dense coarse-grainedgranular soils, cemented zones, and softor weathered rock.
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Examination of the cuttings suspended in the wash fluid provides an opportunity to identify changes in thesoil conditions between sample locations (Figure 3-6d). A strainer is held in the drill fluid discharge streamto catch the suspended material (Figure 3-6e,f). In some instances (especially with uncased holes) the drillfluid return is reduced or lost. This is indicative of open joints, fissures, cavities, gravel layers, highlypermeable zones and other stratigraphic conditions that may cause a sudden loss in pore fluid and must benoted on the logs.
The properties of the drilling fluid and the quantity of water pumped through the bit will determine the sizeof particles that can be removed from the boring with the circulating fluid. In formations containing gravel,cobbles, or larger particles, coarse material may be left in the bottom of the boring. In these instances,clearing the bottom of the boring with a larger-diameter sampler (such as a 76 mm (3.0 in) OD split-barrelsampler) may be needed to obtain a representative sample of the formation.
Note 1: “W” and “X” type rods are the most common types of drill rod and require a separate coupling toconnect rods in series. Other types of rods have been developed for wireline sampling (“WL”) and otherspecific applications. Note 2: Adapted after Boart Longyear Company and Christensen Dia-Min Tools, Inc. For updates, see: http://www.boartlongyear.com/
Note 1: Coupling system is incorporated into casing and are flush, internally and externally.Note 2: Adapted after Boart Longyear Company and Christensen Dia-Min Tools, Inc. For updates, see: http://www.boartlongyear.com/
Figure 3-6. Setup of Bucket Auger & Rig(from ASTM D 4700)
Bucket Auger Borings
Bucket auger drills are used where it is desirable to remove and/or obtain large volumes of disturbed soilsamples, such as for projects where slope stability is an issue. Occasionally, bucket auger borings can beused to make observations of the subsurface by personnel. However this practice is not recommended dueto safety concerns. Video logging provides an effective method for downhole observation.
A common bucket auger drilling configuration is shown in Figure 3-6. Bucket auger borings are usuallydrilled with a 600 mm (24 in) to 1200 mm (48 in) diameter bucket. The bucket length is generally 600 mm(24 in) to 900 mm (36 in) and is basically an open-top metal cylinder having one or more slots cut in its baseto permit the entrance of soil and rock as the bucket is rotated. At the slots, the metal of the base is reinforcedand teeth or sharpened cutting edges are provided to break up the material being sampled.
The boring is advanced by a rotating drilling bucket with cutting teeth mounted to the bottom. The drillingbucket is attached to the bottom of a "kelly bar", which typically consists of two to four square steel tubesassembled one inside another enabling the kelly bar to telescope to the bottom of the hole. At completion ofeach advancement, the bucket is retrieved from the boring and emptied on the ground near the drill rig.
Bucket auger borings are typically advanced by a truck-mounted drill. Small skid-mounted and A-frame drillrigs are available for special uses, such as drilling on steep hillsides or under low clearance (less than 2.5 m(8 ft)). Depending on the size of the rig and subsurface conditions, bucket augers are typically used to drillto depths of about 30 m (100 ft) or less, although large rigs with capabilities to drill to depths of 60 m (200ft) or greater are available.
The bucket auger is appropriate for most soil types and for soft to firm bedrock. Drilling below the watertable can be completed where materials are firm and not prone to large-scale sloughing or water infiltration.For these cases the boring can be advanced by filling it withfluid (water or drilling mud), which provides a positive head andreduces the tendency for wall instability. Manual down-holeinspection and logging should not be performed unless the holeis cased. Only trained personnel should enter a bucket augerboring strict safety procedures established by the appropriateregulatory agencies (e.g. ADSC 1995). Inspection anddownhole logging can more safely be accomplished using videotechniques.
The bucket auger method is particularly useful for drilling inmaterials containing gravel and cobbles because the drillingbucket can auger through cobbles that may cause refusal forconventional drilling equipment. Also, since drilling isadvanced in 300 mm (12 in) to 600 mm (24 in) increments andis emptied after each of these advances, the bucket augeringboring method is advantageous where large-volume samplesfrom specific subsurface locations are required, such as foraggregate studies.
In hard materials (concretions or rocks larger than can enter thebucket), special-purpose buckets and attachments can besubstituted for the standard "digging bucket". Examples of
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special attachments include coring buckets with carbide cutting teeth mounted along the bottom edge, rockbuckets that have heavy-duty digging teeth and wider openings to collect broken materials, single-shankbreaking bars that are attached to the kelly bar and dropped to break up hard rock, and clam shells that areused to pick up cobbles and large rock fragments from the bottom of borings.
Area Specific Methods
Drilling contractors in different parts of the country occasionally develop their own subsurface explorationmethods which may differ significantly from the standard methods or may be a modification of standardmethods. These methods are typically developed to meet the requirements of local site conditions. Forexample, a hammer drill manufactured by Becker Drilling Ltd. of Canada (Becker Hammer) is used topenetrate gravel, dense sand and boulders.
Hand Auger Borings
Hand augers are often used to obtain shallow subsurface information from sites with difficult access or terrainwhere vehicle accessibility is not possible. Several types of hand augers are available with the standard posthole type barrel auger as the most common. In stable cohesive soils, hand augers can be advanced up to 8m (25 ft). Clearly maintaining an open hole in granular soils may be difficult and cobbles & boulders willcreate significant problems. Hand held power augers may be used, but are obviously more difficult to carryinto remote areas. Cuttings contained in the barrel can be logged and tube samples can be advanced at anydepth. Although Shelby tube samples can be taken, small 25- to 50- mm (1.0- to 2.0- inch) diameter tubesare often used to facilitate handling. Other hand auger sampling methods are reviewed in ASTM D 4700.
Exploration Pit Excavation
Exploration pits and trenches permit detailed examination of the soil and rock conditions at shallow depthsand relatively low cost. Exploration pits can be an important part of geotechnical explorations wheresignificant variations in soil conditions occur (vertically and horizontally), large soil and/or non-soil materialsexist (boulders, cobbles, debris) that cannot be sampled with conventional methods, or buried features mustbe identified and/or measured.
Exploration pits are generally excavated with mechanical equipment (backhoe, bulldozer) rather than by handexcavation. The depth of the exploration pit is determined by the exploration requirements, but is typicallyabout 2 m (6.5 ft) to 3 m (10 ft). In areas with high groundwater level, the depth of the pit may be limitedby the water table. Exploration pit excavations are generally unsafe and/or uneconomical at depths greaterthan about 5 m (16 ft) depending on the soil conditions.
During excavation, the bottom of the pit should be kept relatively level so that each lift represents a uniformhorizon of the deposit. At the surface, the excavated material should be placed in an orderly manner adjoiningthe pit with separate stacks to identify the depth of the material. The sides of the pit should be cleaned bychipping continuously in vertical bands, or by other appropriate methods, so as to expose a clean face of rockor soil.
Survey control at exploration pits should be done using optical survey methods to accurately determine theground surface elevation and plan locations of the exploration pit. Measurements should be taken andrecorded documenting the orientation, plan dimensions and depth of the pit, and the depths and the thicknessof each stratum exposed in the pit.
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Exploration pits can, generally, be backfilled with the spoils generated during the excavation. The backfilledmaterial should be compacted to avoid excessive settlements. Tampers or rolling equipment may be used tofacilitate compaction of the backfill.
The U.S. Department of Labor's Construction Safety and Health Regulations, as well as regulations of anyother governing agency must be reviewed and followed prior to excavation of the exploration pit, particularlyin regard to shoring requirements.
Logging Procedures
The appropriate scale to be used in logging the exploration pit will depend on the complexity of geologicstructures revealed in the pit and the size of the pit. The normal scale for detailed logging is 1:20 or 1:10,with no vertical exaggeration.
In logging the exploration pit a vertical profile should be made parallel with one pit wall. The contactsbetween geologic units should be identified and drawn on the profile, and the units sampled (if consideredappropriate by the geotechnical engineer). Characteristics and types of soil or lithologic contacts should benoted. Variations within the geologic units must be described and indicated on the pit log wherever thevariations occur. Sample locations should be shown in the exploration pit log and their locations written ona sample tag showing the station location and elevation. Groundwater should also be noted on the explorationpit log.
Photography and Video Logging
After the pit is logged, the shoring will be removed and the pit may be photographed or video logged at thediscretion of the geotechnical engineer. Photographs and/or video logs should be located with reference toproject stationing and baseline elevation. A visual scale should be included in each photo and video.
3.1.2 Soil Samples
Soil samples obtained for engineering testing and analysis, in general, are of two main categories:
C Disturbed (but representative)
C Undisturbed
Disturbed Samples
Disturbed samples are those obtained using equipment that destroy the macro structure of the soil but do notalter its mineralogical composition. Specimens from these samples can be used for determining the generallithology of soil deposits, for identification of soil components and general classification purposes, fordetermining grain size, Atterberg limits,and compaction characteristics of soils. Disturbed samples can beobtained with a number of different methods as summarized in Table 3-4.
Undisturbed Samples
Undisturbed samples are obtained in clay soil strata for use in laboratory testing to determine the engineeringproperties of those soils. Undisturbed samples of granular soils can be obtained, but often specializedprocedures are required such as freezing or resin impregnation and block or core type sampling. It should be
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noted that the term “undisturbed” soil sample refers to the relative degree of disturbance to the soil’s in-situproperties. Undisturbed samples are obtained with specialized equipment designed to minimize thedisturbance to the in-situ structure and moisture content of the soils. Specimens obtained by undisturbedsampling methods are used to determine the strength, stratification, permeability, density, consolidation,dynamic properties, and other engineering characteristics of soils. Common methods for obtainingundisturbed samples are summarized in Table 3-4.
3.1.3 Soil Samplers
A wide variety of samplers are available to obtain soil samples for geotechnical engineering projects. Theseinclude standard sampling tools which are widely used as well as specialized types which may be unique tocertain regions of the country to accommodate local conditions and preferences. The following discussionsare general guidelines to assist geotechnical engineers and field supervisors select appropriate samplers, butin many instances local practice will control. Following is a discussion of the more commonly used types ofsamplers.
Bulk Disturbed Gravels, Sands, Silts, Clays Hand tools, bucketaugering
<1
Block Undisturbed Cohesive soils and frozen orresin impregnated granular soil
Hand tools <1
Split Barrel SamplerThe split-barrel (or split spoon) sampler is used to obtain disturbed samples in all types of soils. The splitspoon sampler is typically used in conjunction with the Standard Penetration Test (SPT), as specified inAASHTO T206 and ASTM D1586, in which the sampler is driven with a 63.5-kg (140-lb) hammer droppingfrom a height of 760 mm (30 in). Details of the Standard Penetration Test are discussed in Section 5.1.
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(b)(a)
In general, the split-barrel samplers are available in standard lengths of 457 mm (18 in) and 610 mm (24 in)with inside diameters ranging from 38.1 mm (1.5 in) to 114.3 mm (4.5 in) in 12.7 mm (0.5 in) increments(Figure 3-7a,b). The 38.1 mm (1.5 in) inside diameter sampler is popular because correlations have beendeveloped between the number of blows required for penetration and a few select soil properties. The larger-diameter samplers (inside diameter larger than 51 mm (2 in) are sometimes used when gravel particles arepresent or when more material is needed for classification tests.
The 38.1 mm (1.5 in) inside diameter standard split-barrel sampler has an outside diameter of 51 mm (2.0in) and a cutting shoe with an inside diameter of 34.9 mm (1.375 in). This corresponds to a relatively thick-walled sampler with an area ratio [Ar = 100 * (Dexternal
2 - Dinternal2) / Dinternal
2] of 112 percent (Hvorslev, 1949).This high area ratio disturbs the natural characteristics of the soil being sampled, thus disturbed samples areobtained.
A ball check valve incorporated in the sampler head facilitates the recovery of cohesionless materials. Thisvalve seats when the sampler is being withdrawn from the borehole, thereby preventing water pressure on thetop of the sample from pushing it out. If the sample tends to slide out because of its weight, vacuum willdevelop at the top of the sample to retain it.
As shown in Figure 3-8a, when the shoe and the sleeve of this type of sampler are unscrewed from the splitbarrel, the two halves of the barrel may be separated and the sample may be extracted easily. The soil sampleis removed from the split-barrel sampler it is either placed and sealed in a glass jar, sealed in a plastic bag,or sealed in a brass liner (Figure 3-8b). Separate containers should be used if the sample contains differentsoil types. Alternatively, liners may be placed inside the sampler with the same inside diameter as the cuttingshoe (Figure 3-9a). This allows samples to remain intact during transport to the laboratory. In both cases,samples obtained with split barrels are disturbed and therefore are only suitable for soil identification andgeneral classification tests.
Steel or plastic sample retainers are often required to keep samples of clean granular soils in the split-barrelsampler. Figure 3-9b shows a basket shoe retainer, a spring retainer and a trap valve retainer. They areinserted inside the sampler between the shoe and the sample barrel to help retain loose or flowing materials.These retainers permit the soil to enter the sampler during driving but upon withdrawal they close and thereby retain the sample. Use of sample retainers should be noted on the boring log.
Figure 3-7. Split-Barrel Samplers: (a) Lengths of 457 mm (18 in) and 610 mm (24 in); (b) Inside diameters from 38.1 mm (1.5 in) to 89 mm (3.5 in).
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(a) (b)
(a) (b)
Figure 3-8. Split Barrel Sampler: (a) Open sampler with soil sample and cutting shoe; (b) Samplejar, split-spoon, shelby tube, and storage box for transport of jar samples.
In U.S. practice, it is normal to omit the inside liner in the split-spoon barrel. The resistance of thesampler to driving is altered depending upon whether or not a liner is used (Skempton, 1986; Kulhawy& Mayne, 1990). Therefore, in the case that a liner is used, then the boring logs used be clearly notedto reflect this variation from standard U.S. procedures, as the reported numbers in driving may affect theengineering analysis.
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Figure 3-11. Selected Sizes and Types of Thin-Walled Shelby Tubes.
Figure 3-10. Schematic of Thin-Walled Shelby Tube
(After ASTM D 4700).
Thin Wall Sampler
The thin-wall tube (Shelby) sampler is commonly used to obtain relatively undisturbed samples of cohesivesoils for strength and consolidation testing. The sampler commonly used (Figures 3-10) has a 76 mm (3.071in) outside diameter and a 73 mm (2.875 in) inside diameter, resulting in an area ratio of 9 percent. Thinwall samplers vary in outside diameter between 51 mm (2.0 in) and 76 mm (3.0 in) and typically come inlengths from 700 mm (27.56 in) to 900 mm (35.43 in), as shown in Figure 3-11. Larger diameter samplertubes are used where higher quality samples are required and sampling disturbance must be reduced. Thetest method for thin-walled tube sampling is described in AASHTO T 207 and ASTM D 1587.
The thin-walled tubes are manufactured using carbon steel, galvanized-coated carbon steel, stainless steel,and brass. The carbon steel tubes are often the lowest cost tubes but are unsuitable if the samples are to bestored in the tubes for more than a few days or if the inside of the tubes become rusty, significantlyincreasing the friction between the tube and the soil sample. In stiff soils, galvanized carbon steel tubes arepreferred since carbon steel is stronger, less expensive, and galvanizing provides additional resistance tocorrosion. For offshore bridge borings, salt-water conditions, or long storage times, stainless steel tubesare preferred. The thin-walled tube is manufactured with a beveled front edge for cutting a reduced-diametersample [commonly 72 mm (2.835 in) inside diameter] to reduce friction. The thin-wall tubes can be pushedwith a fixed head or piston head, as described later.
The thin-wall tube sampler should not be pushed more than the total length up to the connecting cap less75 mm (3 in). The remaining 75 mm (3 in) of tube length is provided to accommodate the slough thataccumulates to a greater or lesser extent at the bottom of the boring. The sample length is approximately600 mm (24 in). Where low density soils or collapsible materials are being sampled, a reduced push of 300mm (12 in) to 450 mm (18 in) may be appropriate to prevent the disturbance of the sample. The thin-walledtube sampler should be pushed slowly with a single, continuous motion using the drill rig's hydraulicsystem. The hydraulic pressure required to advance the thin-walled tube sampler should be noted andrecorded on the log. The sampler head contains a check valve that allows water to pass through thesampling head into the drill rods. This check valve must be clear of mud and sand and should be checkedprior to each sampling attempt. After the push is completed, the driller should wait at least ten minutes toallow the sample to swell slightly within the tube, then rotate the drill rod string through two completerevolutions to shear off the sample, and then slowly and carefully bring the sample to the surface. In stiffsoils it is often unnecessary to rotate the sampler.
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(b)(a)
After taking a thin-walled tube sample, slough or cuttings from the upper end of the tube should be removedusing a cleanout tool. The length of sample recovered should be measured and the soil classified for the log.About 25-mm of material at the bottom end of the tube should be removed and the cuttings placed in aproperly labeled storage jar. Both ends of the tube should then be sealed with at least a 25 mm (1 in) thicklayer of microcrystalline (nonshrinking) wax after placing a plastic disk to protect the ends of the sample(Figure 3-12a). The remaining void above the top of the sample should be filled with moist sand. Plasticend caps should then be placed over both ends of the tube and electrician's tape placed over the joint betweenthe collar of the cap and the tube and over the screw holes. The capped ends of the tubes are then dipped inmolten wax. Alternatively, O-ring packers can be inserted into the sample ends and then sealed (Figure 3-12b). This may be preferable as it is cleaner and more rapid. In both cases, the sample must be sealed toensure proper preservation of the sample. Samples must be stored upright in a protected environment toprevent freezing, desiccation, and alteration of the moisture content (ASTM D 4220). In some areas of the country, the thin-walled tube samples are field extruded, rather than transported to thelaboratory in the tube. This practice is not recommended due to the uncontrolled conditions typical of fieldoperations, and must not be used if the driller does not have established procedures and equipment forpreservation and transportation of the extruded samples. Rather, the tube sample should be transportedfollowing ASTM D 4220 guidelines to the laboratory and then carefully extruded following a standardizedprocedure.
The following information should be written on the top half of the tube and on the top end cap: projectnumber, boring number, sample number, and depth interval. The field supervisor should also write on thetube the project name and the date the sample was taken. Near the upper end of the tube, the word "top" andan arrow pointing toward the top of the sample should be included. Putting sample information on both thetube and the end cap facilitates retrieval of tubes from laboratory storage and helps prevent mix-ups in thelaboratory when several tubes may have their end caps removed at the same time.
Piston Sampler
The piston sampler (Figure 3-13) is basically a thin-wall tube sampler with a piston, rod, and a modifiedsampler head. This sampler, also known as an Osterberg or Hvorslev sampler, is particularly useful forsampling soft soils where sample recovery is often difficult although it can also be used in stiff soils.
The sampler, with its piston located at the base of the sampling tube, is lowered into the borehole. When thesampler reaches the bottom of the hole, the piston rod is held fixed relative to the ground surface and thethin-wall tube is pushed into the soil slowly by hydraulic pressure or mechanicaljacking. The sampler is never driven. Upon completion of sampling, the sampleris removed from the borehole and the vacuum between the piston and the top of thesample is broken. The piston head and the piston are then removed from the tubeand jar samples are taken from the top and bottom of the sample for identificationpurposes. The tube is then labeled and sealed in the same way as a Shelby tubedescribed in the previous section.
The quality of the samples obtainedis excellent and the probability ofobtaining a satisfactory sample ishigh. One of the major advantages isthat the fixed piston helps prevent theentrance of excess soil at thebeginning of sampling, therebyprecluding recovery ratios greaterthan 100 percent. It also helps thesoil enter the sampler at a constantrate throughout the sampling push.Thus, the opportunity for 100 percentrecovery is increased. The head usedon this sampler also acts creates abetter vacuum which helps retainthe sample better than the ballvalve in thin-walled tube (Shelby)samplers.
Pitcher Tube Sampler
The pitcher tube sampler is used in stiff to hardclays and soft rocks, and is well adapted tosampling deposits consisting of alternately hardand soft layers. This sampler is pictured in Figure3-14 and the primary components shown in Figure3-15a. These include an outer rotating core barrelwith a bit and an inner stationary, spring-loaded,thin-wall sampling tube that leads or trails theouter barrel drilling bit, depending on thehardness of the material being penetrated.
When the drill hole has been cleaned, the sampleris lowered to the bottom of the hole (Figure 3-15a). When the sampler reaches the bottom of thehole, the inner tube meets resistance first and theouter barrel slides past the tube until the spring at the top of the tube contacts the top of the outer barrel. Atthe same time, the sliding valve closes so that the drilling fluid is forced to flow downward in the annularspace between the tube and the outer core barrel and then upward between the sampler and the wall of thehole. If the soil to be penetrated is soft, the spring will compress slightly (Figure 3-15b) and the cutting edgeof the tube will be forced into the soil as downward pressure is applied. This causes the cutting edge to lead
Figure 3-13. Piston Sampler: (a) Picture with thin-walled tube cut-out to show piston; (b) Schematic (After ASTM D 4700).
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Figure 3-15. Pitcher Sampler. (a) Sampler Being Lowered into Drill Hole; (b) SamplerDuring Sampling of Soft Soils; (c) Sampler During Sampling of Stiff or DenseSoils (Courtesy of Mobile Drilling, Inc.).
the bit of the outer core barrel. If the material is hard, the spring compresses a greater amount and the outerbarrel passes the tube so that the bit leads the cutting edge of the tube (Figure 3-15c). The amount by whichthe tube or barrel leads is controlled by the hardness of the material being penetrated. The tube may lead thebarrel by as much as 150 mm (6 in) and the barrel may lead the tube by as much as 12 mm (0.5 in).
Sampling is accomplished by rotating the outer barrel at 100 to 200 revolutions per minute (rpm) whileexerting downward pressure. In soft materials sampling is essentially the same as with a thin-wall samplerand the bit serves merely to remove the material from around the tube. In hard materials the outer barrel cutsa core, which is shaved to the inside diameter of the sample tube by the cutting edge and enters the tube asthe sampler penetrates. In either case, the tube protects the sample from the erosive action of the drillingfluid at the base of the sampler. The filled sampling tube is then removed from the sampler and is marked,preserved, and transported in the same manner described above for thin-walled tubes.
A Denison sampler is similar to a pitcher sampler except that theprojection of the sampler tube ahead of the outer rotating barrel ismanually adjusted before commencement of sampling operations, ratherthan spring-controlled during sampler penetration. The basiccomponents of the sampler (Figure 3-16) are an outer rotating corebarrel with a bit, an inner stationary sample barrel with a cutting shoe,inner and outer barrel heads, an inner barrel liner, and an optionalbasket-type core retainer. The coring bit may either be a carbide insertbit or a hardened steel sawtooth bit. The shoe of the inner barrel has asharp cutting edge. The cutting edge may be made to lead the bit by 12mm (0.5 in) to 75 mm (3 in) through the use of coring bits of differentlengths. The longest lead is used in soft and loose soils because theshoe can easily penetrate these materials and the longer penetration isrequired to provide the soil core with maximum protection againsterosion by the drilling fluid used in the coring. The minimum lead isused in hard materials or soils containing gravel.
The Denison sampler is used primarily in stiff to hard cohesive soilsand in sands, which are not easily sampled with thin-wall samplersowing to the large jacking force required for penetration. Samples ofclean sands may be recovered by using driller’s mud, a vacuum valve,and a basket catch. The sampler is also suitable for sampling soft claysand silts.
Modified California Sampler
The Modified California sampler is a large lined tube sampler used in the Midwest and West, but uncommonin the East and South U.S.A. The sampler is thick-walled (area ratio of 77 percent) with an outside diameterof 64 mm (2.5 in) and an inside diameter of 51 mm (2 in). It has a cutting shoe similar to the split-barrelsampler, but with an inside diameter of generally 49 mm (1.93 in). Four 102-mm (4.0-in) long brass linerswith inside diameters of 49 mm (1.93 in) are used to contain the sample. In the West, the ModifiedCalifornia sampler is driven with standard penetration energy. The unadjusted blow count is recorded onthe boring log. In the Midwest the sampler is generally pushed hydraulically. When pushed, the hydraulicpressure required to advance the Modified California sampler should be noted and recorded on the log. Thedriving resistance obtained using a Modified California sampler is not equal to the standard penetration testresistance and must be adjusted if comparisons are necessary.
Continuous Soil Samplers
Several types of continuous soil samplers have been developed. The conventional continuous samplerconsists of a 1.5 m (5 ft) long thick-walled tube which obtains "continuous" samples of soil as hollow-stemaugers are advanced into soil formations. These systems use bearings or fixed hexagonal rods to restrain orreduce rotation of the continuous sampler as the hollow-stem augers are advanced and the tube is pushed intoundisturbed soil below the augers. Recently, continuous hydraulic push samplers have been developed thatare quick & economical (e.g., Geoprobe, Powerprobe). These samplers have inside diameters ranging from15 mm (0.6 in) to 38.1 mm (1.5 in). A steel mandrel is pushed into the ground at a steady rate and the soilis recovered within disposable plastic liners. These devices typically are stand alone and do not require anydrilling. If hard layers are encountered, a percussive vibrating procedure is used for penetration.
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The continuous samples are generally disturbed and therefore are only appropriate for visual observation,index tests, and classification-type laboratory tests (moisture, density). Continuous samplers have beenshown to work well in most clayey soils and in soils with thin sand layers. Less success is typically observedwhen sampling cohesionless soil below the groundwater level, soft soils, or samples that swell followingsampling although modifications are available to increase sample recovery. Information is limited regardingthe suitability of the continuous samples for strength and consolidation tests and therefore must be used withcaution.
Other Soil Samplers
A variety of special samplers are available to obtain samples of soil and soft rocks. These methods includethe retractable plug, Sherbrooke, and Laval samplers. These sampling methods are used in difficult soilswhere the more routine methods do not recover samples.
Bulk Samples
Bulk samples are suitable for soil classification, index testing, R-value, compaction, California Bearing Ratio(CBR), and tests used to quantify the properties of compacted geomaterials. The bulk samples may beobtained using hand tools without any precautions to minimize sample disturbance. The sample may betaken from the base or walls of a test pit or a trench, from drill cuttings, from a hole dug with a shovel andother hand tools, by backhoe, or from a stockpile. The sample should be put into a container that will retainall of the particle sizes. For large samples, plastic or metal buckets or metal barrels are used; for smallersamples, heavy plastic bags that can be sealed to maintain the water content of the samples are used.
Usually, the bulk sample provides representative materials that will serve as borrow for controlled fill inconstruction. Laboratory testing for soil properties will then rely on compacted specimens. If the materialis relatively homogeneous, then bulk samples may be taken equally well by hand or by machine. However,in stratified materials, hand excavation may be required. In the sampling of such materials it is necessaryto consider the manner in which the material will be excavated for construction. If it is likely that thematerial will be removed layer by layer through the use of scrapers, samples of each individual material willbe required and hand excavation from base or wall of the pit may be a necessity to prevent unwanted mixingof the soils. If, on the other hand, the material is to be excavated from a vertical face, then the sampling mustbe done in a manner that will produce a mixture having the same relative amounts of each layer as will beobtained during the borrow area excavation. This can usually be accomplished by hand-excavating a shallowtrench down the walls of the test pit within the depth range of the materials to be mixed.
Block Samples
For projects where the determination of the undisturbed properties is very critical, and where the soil layersof interest are accessible, undisturbed block samples can be of great value. Of all the undisturbed testingmethods discussed in this manual, properly-obtained block samples produce samples with the least amountof disturbance. Such samples can be obtained from the hillsides, cuts, test pits, tunnel walls and otherexposed sidewalls. Undisturbed block sampling is limited to cohesive soils and rocks. The procedures usedfor obtaining undisturbed samples vary from cutting large blocks of soil using a combination of shovels, handtools and wire saws, to using small knives and spatulas to obtain small blocks.
In addition, special down-hole block sampling methods have been developed to better obtain samples in theirin-situ condition. For cohesive soils, the Sherbrooke sampler has been developed and is able to obtainsamples 250 mm (9.85 in) diameter and 350 mm (13.78 in) height (Lefebvre and Poulin 1979). In-situfreezing methods for saturated granular soils and resin impregnation methods have been implemented to“lock” the soil in the in-situ condition prior to sampling. When implemented, these methods have been
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shown to produce high quality undisturbed samples. However, the methods are rather involved and timeconsuming and therefore have not seen widespread use in practice.
Once samples are obtained and transported to the laboratory in suitable containers, they are trimmed toappropriate size and shape for testing. Block samples should be wrapped with a household plastic membraneand heavy duty foil and stored in block form and only trimmed shortly before testing. Every sample mustbe identified with the following information: project number, boring or exploration pit number, samplenumber, sample depth, and orientation.
3.1.4 Sampling Interval and Appropriate Type of Sampler
In general, SPT samples are taken in both granular and cohesive soils, and thin-walled tube samples are takenin cohesive soils. The sampling interval will vary between individual projects and between regions. Acommon practice is to obtain split barrel samples at 0.75 m (2.5 ft) intervals in the upper 3 m (10 ft) and at1.5 m (5 ft) intervals below 3 m (10 ft). In some instances, a greater sample interval, often 3 m (10 ft), isallowed below depths of 30 m (100 ft). In other cases, continuous samples may be required for some portionof the boring.
In cohesive soils, at least one undisturbed soil sample should be obtained from each different stratumencountered. If a uniform cohesive soil deposit extends for a considerable depth, additional undisturbedsamples are commonly obtained at 3 m (10 ft) to 6 m (10 ft) intervals. Where borings are widely spaced, itmay be appropriate to obtain undisturbed samples in each boring; however, for closely spaced borings, orin deposits which are generally uniform in lateral extent, undisturbed samples are commonly obtained onlyin selected borings. In erratic geologic formations or thin clay layers it is sometimes necessary to drill aseparate boring adjacent to a previously completed boring to obtain an undisturbed sample from a specificdepth which may have been missed in the first boring.
3.1.5 Sample Recovery
Occasionally, sampling is attempted and little or no material is recovered. In cases where a split barrel, oran other disturbed-type sample is to be obtained, it is appropriate to make a second attempt to recover thesoil sample immediately following the first failed attempt. In such instances, the sampling device is oftenmodified to include a retainer basket, a hinged trap valve, or other measures to help retain the material withinthe sampler.
In cases where an undisturbed sample is desired, the field supervisor should direct the driller to drill to thebottom of the attempted sampling interval and repeat the sampling attempt. The method of sampling shouldbe reviewed, and the sampling equipment should be checked to understand why no sample was recovered(such as a plugged ball valve). It may be appropriate to change the sampling method and/or the samplingequipment, such as waiting a longer period of time before extracting the sampler, extracting the sampler moreslowly and with greater care, etc. This process should be repeated or a second boring may be advanced toobtain a sample at the same depth.
3.1.6 Sample Identification
Every sample which is attempted, whether recovered or not, should be assigned a unique number composedof designators for the project number or name, boring number, sequential sample attempt number, andsample depth. Where tube samples are obtained, any disturbed tubes should be clearly marked with thesample identification number and the top and bottom of the sample labeled.
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3.1.7 Relative Strength Tests
In addition to the visual observations of soil consistency, a pocket (hand) penetrometer can be used toestimate the strength of soil samples. The hand penetrometer estimates the unconfined strength and issuitable for firm to very stiff clay soils. A larger foot/adaptor is needed to test softer soils. It should beemphasized that this test does not produce absolute values; rather it should be used as a guide in estimatingthe relative strength of soils. Values obtained with a hand penetrometer should not be used in design.Instead, when the strength of soils (and other engineering properties) is required, in-situ tests and/or a seriesof laboratory tests (as described in Chapter 7) on undistrubed samples should be performed.
Another useful test device is a torvane, which is a small diameter vane shear testing device that provides anestimate of the shear strength of cohesive soils. Variable diameter vanes are available for use in very softto very stiff cohesive soils. Again, this field test yields values that can be used for comparison purposes only,and the torvane results should not be used in any geotechnical engineering analysis or design.
Testing with a penetrometer or torvane should always be done in natural soils as near as possible to the centerof the top or bottom end of the sample. Testing on the sides of extruded samples is not acceptable. Strengthvalues obtained from pocket penetrometer or torvane should not be used for design purposes.
3.1.8 Care and Preservation of Undisturbed Soil Samples
Each step in sampling, extruding, storing and testing introduces varying degrees of disturbance to the sample.Proper sampling, handling, and storage methods are essential to minimize disturbances. The geotechnicalengineer must be cognizant of disturbance introduced during the various steps in sampling through testing.The field supervisors should be sensitized about disturbance and the consequences. A detailed discussionof sample preservation and transportation is presented in ASTM D 4220 along with a recommendedtransportation container design.
When tube samples are to be obtained, each tube should be examined to assure that it is not bent, that thecutting edges are not damaged, and that the interior of the tubes are not corroded. If the tube walls arecorroded or irregular, or if samples are stored in tubes for long periods of time, the force required to extractthe samples sometimes may exceed the shear strength of the sample causing increased sample disturbance.
All samples should be protected from extreme temperatures. Samples should be kept out of direct sunlightand should be covered with wet burlap or other material in hot weather. In winter months, specialprecautions should be taken to prevent samples from freezing during handling, shipping and storage. Asmuch as is practical, the thin-walled tubes should be kept vertical, with the top of the sample oriented in theup position. If available, the thin-walled tubes should be kept in a carrier with an individual slot for eachtube. Padding should be placed below and between the tubes to cushion the tubes and to prevent them fromstriking one another. The entire carrier should be secured with rope or cable to the body of the transportingvehicle so that the entire case will not tilt or tip over while the vehicle is in motion.
Soil sample extrusion from tubes in the field is an undesired practice and often results in sample swellingand an unnecessary high degree of disturbance. The stress relief undoubtably allows the specimens to softenand expand. The samples are also more susceptible to handling disturbances during transport to thelaboratory. High-quality specimens are best obtained by soil extraction from tubes in the laboratory just priorto consolidation, triaxial, direct shear, permeability, and resonant column testing. However, to save money,some organizations extrude samples in the field in order to re-use the tubes and these samples are oftenwrapped in aluminum foil. Depending on the pH of the soil, the aluminum foil may react with the surfaceof the soil and develop a thin layer of discolored soil, thus making visual identification difficult andconfusing. It may also result in changes in the moisture distribution across the sample. Even though plastic
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sheeting is also susceptible to reacting with the soil contacted, past observation shows that plastic has lesseffect than foil. Thus it is recommended that extruded soil samples which are to be preserved be wrappedin plastic sheeting and then wrapped with foil. However, if possible, samples should not be extracted fromtubes in the field in order to minimize swelling, disturbance, transport, and handling issues.
Storage of undisturbed samples (in or out of tubes) for long periods of time under any condition is notrecommended. Storage exceeding one month may substantially alter soil strength & compressibility asmeasured by lab tests.
3.2 EXPLORATION OF ROCK
The methods used for exploration and investigation of rock include:
C DrillingC Exploration pits (test pits)C Geologic mappingC Geophysical methods
Core drilling which is used to obtain intact samples of rock for testing purposes and for assessing rockquality and structure, is the primary investigative method. Test pits, non-core drilling, and geophysicalmethods are often used to identify the top of rock.
Geophysical methods such as seismic refraction and ground penetrating radar (GPR) may be used to obtainthe depth to rock. Finally, geologic mapping of rock exposures or outcrops provides a means for assessingthe composition and discontinuities of rock strata on a large scale which may be valuable for manyengineering applications particularly rock slope design. This section contains a discussion of drilling andgeologic mapping. Some geophysical methods are discussed in section 5.7.
3.2.1 Rock Drilling and Sampling
Where borings must extend into weathered and unweathered rock formations, rock drilling and samplingprocedures are required. The use of ISRM (International Society for Rock Mechanics) Commission onStandardization of Laboratory and Field Tests (1978, 1981) guidelines are recommended for detailedguidance for rock drilling, coring, sampling, and logging of boreholes in rock masses. This section providesan abbreviated discussion of rock drilling and sampling methods.
Defining the top of rock from drilling operations can be difficult, especially where large boulders exist,below irregular residual soil profiles, and in karst terrain. In all cases, the determination of the top of rockmust be done with care, as an improper identification of the top of rock may lead to miscalculated rockexcavation volume or erroneous pile length. As per ASTM D 2113, core drilling procedures are used whenformations are encountered that are too hard to be sampled by soil sampling methods. A penetration of 25mm (1 in) or less by a 51 mm (2 in) diameter split-barrel sampler following 50 blows using standardpenetration energy or other criteria established by the geologist or engineer should indicate that soil samplingmethods are not applicable and rock drilling or coring is required. In many instances, geophysical methods,such as seismic refraction, can be used to assist in evaluating the top of rock elevations in an expedient andeconomical manner. The refraction data can also provide information between confirmatory boringlocations.
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3.2.2 Non-Core (Destructive) Drilling
Non-core rock drilling is a relatively quick and inexpensive means of advancing a boring which can beconsidered when an intact rock sample is not required. Non-core drilling is typically used for determiningthe top of rock and is useful in solution cavity identification in karstic terrain. Types of non-core drillinginclude air-track drilling, down-the-hole percussive drilling, rotary tricone (roller bit) drilling, rotary dragbit drilling, and augering with carbide-tipped bits in very soft rocks. Drilling fluid may be water, mud,foam, or compressed air. Caution should be exercised when using these methods to define the top of softrock since drilling proceeds rapidly, and cuts weathered and soft rock easily, frequently misrepresentingthe top of rock for elevation or pile driving applications.
Because intact rock samples are not recovered in non-core drilling, it is particularly important for the fieldsupervisor to carefully record observations during drilling. The following information pertaining to drillingcharacteristics should be recorded in the remarks section of the boring log:
C Penetration rate or drilling speed in minutes per 0.3 meter (1 ft)C Dropping of rodsC Changes in drill operation by driller (down pressures, rotation speeds, etc.)C Changes in drill bit conditionC Unusual drilling action (chatter, bouncing, binding, sudden drop)C Loss of drilling fluid, color change of fluid, or change in drilling pressure
3.2.3 Types of Core Drilling
A detailed discussion of diamond core drilling is presented in AASHTO T 225 and ASTM D 2113. Typesof core barrels may be single-tube, double-tube, or triple-tube, as shown in Figures 3-17a,b,c. Table 3-5presents various types of core barrels available on the market. The standard is a double-tube core barrel,which offers better recovery by isolating the rock core from the drilling fluid stream and consists of an innerand outer core barrel as pictured in Figure 3-18. The inner tube can be rigid or fixed to the core barrel headand rotate around the core or it can be mounted on roller bearings which allow the inner tube to remainstationary while the outer tube rotates. The second or swivel type core barrel is less disturbing to the coreas it enters the inner barrel and is useful in coring fractured and friable rock. In some regions only tripletube core barrels are used in rock coring. In a multi-tube system, the inner tube may be longitudinally splitto allow observation and removal of the core with reduced disturbance.
Rock coring can be accomplished with either conventional or wireline equipment. With conventionaldrilling equipment, the entire string of rods and core barrel are brought to the surface after each core runto retrieve the rock core. Wireline drilling equipment allows the inner tube to be uncoupled from the outertube and raised rapidly to the surface by means of a wire line hoist. The main advantage of wireline drillingover conventional drilling is the increased drilling production resulting from the rapid removal of the corefrom the hole which, in turn, decreases labor costs. It also provides improved quality of recovered core,particularly in soft rock, since this method avoids rough handling of the core barrel during retrieval of thebarrel from the borehole and when the core barrel is opened. (Drillers often hammer on the core barrel tobreak it from the drill rods and to open the core barrel, causing the core to break.) Wireline drilling can beused on any rock coring job, but typically, it is used on projects where bore holes are greater than 25 m deepand rapid removal of the core from the hole has a greater effect on cost. Wireline drilling is also an effectivemethod for both rock and soil exploration though cobbles or boulders, which tend to shift and block off thebore hole.
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(a)
(c)
(b)
Figure 3-17. (a) Single Tube Core Barrel; (b) Rigid Type Double Tube Core Barrel; (c) Swivel Type DoubleTube Core Barrel, Series “M” with Ball Bearings (Courtesy of Sprague & Henwood, Inc.).
TABLE 3-5.
DIMENSIONS OF CORE SIZES
(after Christensen Dia-Min Tools, Inc.)Size Diameter of Core
Figure 3-19. Coring Bits. From left to right:Diamond, Carbide, & Sawtooth.
(a)
Although NX is the size most frequently used for engineering explorations, larger and smaller sizes are inuse. Generally, a larger core size will produce greater recovery and less mechanical breakage. Because oftheir effect on core recovery, the size and type of coring equipment used should be carefully recorded in theappropriate places on the boring log.
The length of each core run should be limited to 3 m maximum. Core run lengths should be reduced to 1.5m (5 ft), or less, just below the rock surface and in highly fractured or weathered rock zones. Shorter coreruns often reduce the degree of damage to the coreand improve core recovery in poor quality rock.
Coring Bits
The coring bit is the bottommost component of thecore barrel assembly. It is the grinding action ofthis component that cuts the core from the rockmass. Three basic categories of bits are in use:diamond, carbide insert, and sawtooth (Figure 3-19). Coring bits are generally selected by the drillerand are often approved by the geotechnicalengineer. Bit selection should be based on generalknowledge of drill bit performance for the expectedformations and the proposed drilling fluid.
Diamond coring bits which may be of surface set orimpregnated-diamond type are the most versatilesince they can produce high-quality cores in rock materials ranging from soft to extremely hard. Comparedto other types, diamond bits in general permit more rapid coring and as noted by Hvorslev (1949), exert lowertorsional stresses on the core. Lower torsional stresses permit the retrieval of longer cores and cores of smalldiameter. The wide variation in the hardness, abrasiveness, and degree of fracturing encountered in rock hasled to the design of bits to meet specific conditions known to exist or encountered at given sites. Thus, widevariations in the quality, size, and spacing of diamonds, in the composition of the metal matrix, in the face
contour, and in the type and number of waterways are found in bits of this type. Similarly, the diamondcontent and the composition of the metal matrix of impregnated bits are varied to meet differing rockconditions.
Carbide bits use tungsten carbide in lieu of diamonds and are of several types (the standard type is shown inFigure 3-19). Bits of this type are used to core soft to medium-hard rock. They are less expensive thandiamond bits. However, the rate of drilling is slower than with diamond bits.
Sawtooth bits consist of teeth cut into the bottom of the bit. The teeth are faced and tipped with a hard metalalloy such as tungsten carbide to provide water resistance and thereby to increase the life of the bit. Althoughthese bits are less expensive than diamond bits, they do not provide as high a rate of coring and do not havea salvage value. The saw tooth bit is used primarily to core overburden and very soft rock.
An important feature of all bits which should be noted is the type of waterways provided in the bits forpassage of drilling fluid. Bits are available with so-called “conventional” waterways, which are passages cuton the interior face of the bit), or with bottom discharge waterways, which are internal and discharge at thebottom face of the bit behind a metal skirt separating the core from the discharge fluid. Bottom dischargebits should be used when coring soft rock or rock having soil-filled joints to prevent erosion of the core bythe drilling fluid before the core enters the core barrel.
Drilling Fluid
In many instances, clear water is used as the drilling fluid in rock coring. If drilling mud is required tostabilize collapsing holes or to seal zones when there is loss of drill water, the design engineer, the geologistand the geotechnical engineer should be notified to confirm that the type of drilling mud is acceptable.Drilling mud will clog open joints and fractures, which adversely affects permeability measurements andpiezometer installations. Drilling fluid should be contained in a settling basin to remove drill cuttings andto allow recirculation of the fluid. Generally, drilling fluids can be discharged onto the ground surface.However, special precautions or handling may be required if the material is contaminated with oil or othersubstances and may require disposal off site. Water flow over the ground surface should be avoided, as muchas possible.
3.2.4 Observation During Core Drilling
Drilling Rate/Time
The drilling rate should be monitored and recorded on the boring log in the units of minutes per 0.3 m (1 ft).Only time spent advancing the boring should be used to determine the drilling rate.
Core Photographs
Cores in the split core barrel should be photographed immediately upon removal from the borehole. A labelshould be included in the photograph to identify the borehole, the depth interval and the number of the coreruns. It may be desirable to get a "close-up" of interesting features in the core. Wetting the surface of therecovered core using a spray bottle and/or sponge prior to photographing will often enhance the colorcontrasts of the core.
A tape measure or ruler should be placed across the top or bottom edge of the box to provide a scale in thephotograph. The tape or ruler should be at least 1 meter (3 ft) long, and it should have relatively large, highcontrast markings to be visible in the photograph.
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A color bar chart is often desirable in the photograph to provide indications of the effects of variation in filmage, film processing, and the ambient light source. The photographer should strive to maintain uniform lightconditions from day to day, and those lighting conditions should be compatible with the type of film selectedfor the project.
Rock Classification
The rock type and its inherent discontinuities, joints, seams, and other facets should be documented. SeeSection 4.7 for a discussion of rock classification and other information to be recorded for rock core.
Recovery
The core recovery is the length of rock core recovered from a core run, and the recovery ratio is the ratio ofthe length of core recovered to the total length of the core drilled on a given run, expressed as either a fractionor a percentage. Core length should be measured along the core centerline. When the recovery is less thanthe length of the core run, the non-recovered section should be assumed to be at the end of the run unlessthere is reason to suspect otherwise (e.g., weathered zone, drop of rods, plugging during drilling, loss of fluid,and rolled or recut pieces of core). Non-recovery should be marked as NCR (no core recovery) on the boringlog, and entries should not be made for bedding, fracturing, or weathering in that interval.
Recoveries greater than 100 percent may occur if core that was not recovered during a run is subsequentlyrecovered in a later run. These should be recorded and adjustments to data should not be made in the field.
Rock Quality Designation (RQD)
The RQD is a modified core recovery percentage in which the lengths of all pieces of sound core over 100mm (4 in) long are summed and divided by the length of the core run. The correct procedure for measuringRQD is illustrated in Figure 3-20. The RQD is an index of rock quality in that problematic rock that is highlyweathered, soft, fractured, sheared, and jointed typically yields lower RQD values. Thus, RQD is simply ameasurement of the percentage of "good" rock recovered from an interval of a borehole. It should be notedthat the original correlation for RQD (Rock Quality Designation) reported by Deere (1963) was based onmeasurements made on NX-size core. Experience in recent years reported by Deere and Deere (1989)indicates that cores with diameters both slightly larger and smaller than NX may be used for computing RQD.The wire line cores using NQ, HQ, and PQ are also considered acceptable. The smaller BQ and BX sizesare discouraged because of a higher potential for core breakage and loss.
Length Measurements of Core Pieces
The same piece of core could be measured three ways: along the centerline, from tip to tip, or along the fullycircular barrel section (Figure 3-21). The recommended procedure is to measure the core length along thecenterline. This method is advocated by the International Society for Rock Mechanics (ISRM), Commissionon Standardization of Laboratory and Field Tests (1978, 1981). The centerline measurement is preferredbecause: (1) it results in a standardized RQD not dependent on the core diameter, and (2) it avoids undulypenalizing of the rock quality for cases where the fractures parallel the borehole and are cut by a second set.
Core breaks caused by the drilling process should be fitted together and counted as one piece. Drilling breaksare usually evidenced by rough fresh surfaces. For schistose and laminated rocks, it is often difficult todiscern the difference between natural breaks and drilling breaks. When in doubt about a break, it should beconsidered as natural in order to be conservative in the calculation of RQD for most uses. It is noted that thispractice would not be conservative when the RQD is used as part of a ripping or dredging estimate.
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Figure 3-20. Modified Core Recovery as an Index of Rock Mass Quality.
Assessment of Soundness
Pieces of core which are not "hard and sound" should not be counted for the RQD even though they possessthe requisite 100 mm (3.94 in) length. The purpose of the soundness requirement is to downgrade the rockquality where the rock has been altered and weakened either by agents of surface weathering or byhydrothermal activity. Obviously, in many instances, a judgment decision must be made as to whether ornot the degree of chemical alteration is sufficient to reject the core piece.
One commonly used procedure is not to count a piece of core if there is any doubt about its meeting thesoundness requirement (because of discolored or bleached grains, heavy staining, pitting, or weak grainboundaries). This procedure may unduly penalize the rock quality, but it errs on the side of conservatism.A second procedure which occasionally has been used is to include the altered rock within the RQD summedpercentage, but to indicate by means of an asterisk (RQD*) that the soundness requirements have not beenmet. The advantage of the method is that the RQD* will provide some indication of the rock quality withrespect to the degree of fracturing, while also noting its lack of soundness.
3 - 29
Figure 3-21. Length Measurement of Core RQD Determination.
Drilling Fluid Recovery
The loss of drilling fluid during the advancement of a boring can be indicative of the presence of open joints,fracture zones or voids in the rock mass being drilled. Therefore, the volumes of fluid losses and the intervalsover which they occur should be recorded. For example, "no fluid loss" means that no fluid was lost exceptthrough spillage and filling the hole. "Partial fluid loss" means that a return was achieved, but the amount of return was significantly less than the amount being pumped in. "Complete water loss" meansthat no fluid returned to the surface during the pumping operation. A combination of opinions from the fieldpersonnel and the driller on this matter will result in the best estimate.
Core Handling and Labeling
Rock cores from geotechnical explorations should be stored in structurally sound core boxes made of woodor corrugated waxed cardboard (Figure 3-22). Wooden boxes should be provided with hinged lids, with thehinges on the upper side of the box and a latch to secure the lid in a closed position.
Cores should be handled carefully during transfer from barrel to box to preserve mating across fractures andfracture-filling materials. Breaks in core that occur during or after the core is transferred to the core box
3 - 30
Figure 3-22. Core Box for Storage of Recovered Rock and Labeling.
should be refitted and marked with three short parallel lines across the fracture trace to indicate a mechanicalbreak. Breaks made to fit the core into the core box and breaks made to examine an inner core surface shouldbe marked as such. These deliberate breaks should be avoided unless absolutely necessary. Cores shouldbe placed in the boxes from left to right, top to bottom. When the upper compartment of the box is filled,the next lower (or adjoining) compartment (and so on until the box is filled) should be filled, beginning ineach case at the left-hand side. The depths of the top and bottom of the core and each noticeable gap in theformation should be marked by a clearly labeled wooden spacer block.
If there is less than 100 percent core recovery for a run, a cardboard tube spacer of the same length as the coreloss should be placed in the core box either at the depth of core loss, if known, or at the bottom of the run.The depth of core loss, if known, or length of core loss should be marked on the spacer with a blackpermanent marker. The core box labels should be completed using an indelible black marking pen. Anexample of recommended core box markings is given in Figure 3-22. The core box lid should have identicalmarkings both inside and out, and both exterior ends of the box should be marked as shown. For angledborings, depths marked on core boxes and boring logs should be those measured along the axis of the boring.The angle and orientation of the boring should be noted on the core box and the boring log.
3 - 31
Care and Preservation of Rock Samples
A detailed discussion of sample preservation and transportation is presented in ASTM D 5079. Four levelsof sample protection are identified:
C Routine careC Special careC Soil-like careC Critical care
Most geotechnical explorations will use routine care in placing rock core in core boxes. ASTM D 5079suggests enclosing the core in a loose-fitting polyethylene sleeve prior to placing the core in the core box.Special care is considered appropriate if the moisture state of the rock core (especially shale, claystone andsiltstone) and the corresponding properties of the core may be affected by exposure. This same procedurecan also apply if it is important to maintain fluids other than water in the sample. Critical care is needed toprotect samples against shock and vibration or variations in temperature, or both. For soil-like care, samplesshould be treated as indicated in ASTM D 4220.
Figure 3-23. Rock Formations Showing Joints,Cut Slopes, Planes, and Stabilization Measures.
3.2.5 Geologic Mapping
Geologic mapping is briefly discussed here, with a more thorough review in FHWA Module 5 (Rock Slopes).Geologic mapping is the systematic collection of local, detailed geologic data, and, for engineering purposes,is used to characterize and document the condition of a rock mass or outcrop. The data derived fromgeologic mapping is a portion of the data required for design of a cut slope or for stabilization of an existingslope. Geologic mapping can often provide more extensive and less costly information than drilling. Theguidelines presented are intended for rock and rock-like materials. Soil and soil-like materials, althoughoccasionally mapped, are not considered in this section.
Qualified personnel trained in geology or engineering geology should perform the mapping or providesupervision and be responsible for the mapping activities and data collection. The first step in geologicmapping is to review and become familiar with the local and regional geology from published and non-published reports, maps and investigations. The mapping team should be knowledgeable of the rock units
3 - 32
and structural and historical geologic aspects of the area. A team approach (minimum of two people, the“buddy system”) is recommended for mapping as a safety precaution when mapping in isolated areas.
Procedures for mapping are outlined in an FHWA Manual (1989) on rock slope design, excavation andstabilization and in ASTM D 4879. The first reference describes the parameters to be considered whenmapping for cut slope design, which include:
C Discontinuity typeC Discontinuity orientationC Discontinuity in fillingC Surface propertiesC Discontinuity spacingC PersistenceC Other rock mass parameters
These parameters can be easily recorded on a structural mapping coding form shown in Figure 3-24. ASTMD 4879 also describes similar parameters and presents commonly used geologic symbols for mappingpurposes. It also presents a suggested report outline. Presentation of discontinuity orientation data can begraphically plotted using stereographic projections. These projections are very useful in rock slope stabilityanalyses. Chapter 3 (Graphical presentation of geological data) in the FHWA manual cited above describesthe stereographic projection methods in detail.
3.3 BORING CLOSURE
All borings should be properly closed at the completion of the field exploration. This is typically requiredfor safety considerations and to prevent cross contamination of soil strata and groundwater. Boring closureis particularly important for tunnel projects since an open borehole exposed during tunneling may lead touncontrolled inflow of water or escape of compressed air.
In many parts of the country, methods to be used for the closure of boreholes are regulated by state agencies.National Cooperative Highway Research Program Report No. 378 (1995) titled “Recommended Guidelinesfor Sealing Geotechnical Holes” contains extensive information on sealing and grouting. The regulationsin general, require that any time groundwater or contamination is encountered the borehole be grouted usinga mixture of powdered bentonite, Portland cement and potable water. Some state agencies require groutingof all boreholes exceeding a certain depth. The geotechnical engineer and the field supervisor should beknowledgeable about local requirements prior to commencing the borings.
It is good practice to grout all boreholes. Holes in pavements and slabs should be filled with quick settingconcrete, or with asphaltic concrete, as appropriate. Backfilling of boreholes is generally accomplished usinga grout mixture . The grout mix is normally pumped though drill rods or other pipes inserted into theborehole. In boreholes filled with water or other drilling fluids the tremied grout will displace the drill fluid.Provisions should be made to collect and dispose of all displaced drill fluid and waste grout.
3.4 SAFETY GUIDELINES FOR GEOTECHNICAL BORINGS
All field personnel, including geologists, engineers, technicians, and drill crews, should be familiar with thegeneral health and safety procedures, as well as any additional requirements of the project or governingagency.
3 - 3
3 Day
Mon
th
Yea
r
Dis
cont
inui
ty d
ata
Dat
eIn
spec
tor
Shee
t No.
ofN
ATU
RE
AN
D O
RIE
NTA
TIO
N O
F D
ISC
ON
TIN
UIT
YSt
atio
nor
No.
Type
Dip
Dip
dire
ctio
nPe
rsis
tenc
eA
pertu
reN
atur
e of
filli
ngSt
reng
th o
ffil
ling
Surf
ace
Rou
ghne
ssTr
end
oflin
eatio
nW
avin
ess
wav
elen
gth
Wav
ines
sam
plitu
de W
ater
/Fl
owR
emar
ks
Type
Pers
iste
nce
Ape
rture
Nat
ure
of fi
lling
Com
pres
sive
stre
ngth
of
infil
ling
Rou
ghne
ssW
ater
Flo
w (I
nfill
ed)
Wat
er F
low
(Fill
ed)
0. F
ault
zone
1. F
ault
2. J
oint
3. C
leav
age
4. S
chis
tosi
ty5.
She
ar6.
Fis
sure
7. T
ensi
on c
rack
8. F
olia
tion
9. B
eddi
ng
1. V
ery
low
per
sist
ence
2. L
ow P
ersi
sten
ce3.
Med
ium
per
sist
ence
4. H
igh
pers
iste
nce
5. V
ery
high
per
sist
ence
<1 m
1-3
m3-
10 m
10-2
0 m
>20
m
1. V
ery
tight
(<
0.1
mm
)2.
Tig
ht
(0.1
-0.2
5 m
m)
3. P
artly
ope
n
(0.2
5-0.
5 m
m)
4. O
pen
(0
.5-2
.5 m
m)
5. M
oder
atel
y
wid
e
(2.5
-10
mm
)6.
Wid
e
(>10
mm
)7.
Ver
y w
ide
(1
-10
cm)
8. E
xtre
mel
y w
ide
(1
0 - 1
00 c
m)
9. C
aver
nous
1. C
lean
2. S
urfa
ce st
aini
ng3.
Non
-coh
esiv
e4.
Ina
ctiv
e cl
ay
or c
lay
mat
rix5.
Sw
ellin
g cl
ay
or
cla
y m
atrix
6. C
emen
ted
7. C
hlor
ite, t
ale,
or
gyp
sum
8. O
ther
s - sp
ecify
1.
Ver
y st
rong
rock
2.
Stro
ng ro
ck3.
M
oder
atel
y st
rong
rock
4.
Mod
erat
ely
wea
k ro
ck5.
V
ery
wea
k ro
ck6.
V
ery
stiff
soil
7.
Stiff
soil
8.
Firm
soil
9.
Soft
soil
10.
Ver
y so
ft so
il
MPa
>200
100-
200
50-1
0025
-50
1-25
0.6-
1.0
0.15
-0.6
0.08
-0.1
50.
04-0
.08
<0.0
4
1. R
ough
(or i
rreg
ular
),
st
eppe
d2.
Sm
ooth
, ste
pped
3. S
licke
nsid
ed st
eppe
d4.
Rou
gh (o
r irr
egul
ar),
u
ndul
atin
g5.
Sm
ooth
, und
ulat
ing
6. S
licke
nsid
ed,
und
ulat
ing
7. R
ough
(or i
rreg
ular
),
pl
anar
8. S
moo
th, p
lana
r9.
Slic
kens
ided
Seep
age
Rat
ing
I II III
IV V VI
Des
crip
tion
The
disc
ontin
uity
is v
ery
tight
and
dry
; wat
er fl
owal
ong
it do
es n
ot a
ppea
rpo
ssib
le.
The
disc
ontin
uity
is d
ryw
ith n
o ev
iden
ce o
f wat
erflo
w.
The
disc
ontin
uity
is d
ry b
utsh
ows e
vide
nce
of w
ater
flow
, i.e
. rus
t sta
inin
g, e
tc.
The
disc
ontin
uity
is d
amp
but n
o fr
ee w
ater
ispr
esen
t.Th
e di
scon
tinui
ty sh
ows
seep
age,
occ
asio
nal d
rops
of w
ater
, but
no
cont
inuo
usflo
w.
The
disc
ontin
uity
show
s aco
ntin
uous
flow
of w
ater
(Est
imat
e l/m
in a
ndde
scrib
e pr
essu
re, i
.e. l
ow,
med
ium
, hig
h).
W1
W2
W3
W4
W5
The
fillin
g m
ater
ials
are
heav
ily c
onso
lidat
ed a
nddr
y; si
gnifi
cant
flow
appe
ars u
nlik
ely
due
tove
ry lo
w p
erm
eabi
lity.
The
fillin
g m
ater
ials
are
dam
p, b
ut n
o fr
ee w
ater
ispr
esen
t.Th
e fil
ling
mat
eria
ls a
rew
et; o
ccas
iona
l dro
ps o
fw
ater
.Th
e fil
ling
mat
eria
ls sh
owsi
gns o
f out
was
h,co
ntin
uous
flow
of w
ater
(est
imat
e lit
ers/
min
ute)
.Th
e fil
ling
mat
eria
ls a
rew
ashe
d ou
t loc
ally
;co
nsid
erab
le w
ater
flow
alon
g ou
t-was
h ch
anne
ls(e
stim
ate
liter
s/m
inut
e an
dde
scrib
e pr
essu
re, i
.e. l
ow,
med
ium
, hig
h).
Figu
re 3
-24.
(a) S
truc
tura
l Map
ping
Cod
ing
Form
for
Dis
cont
inui
ty S
urve
y D
ata.
3 - 3
4
Figu
re 3
-24.
(b) S
truc
tura
l Map
ping
Cod
ing
Form
for
Slop
e A
sses
smen
t.
3 - 35
Typical safety guidelines for drilling into soil and rock are presented in Appendix A. Minimum protectivegear for all personnel should include hard hat, safety boots, eye protection, and gloves.
It is not unusual to encounter unknown or unexpected environmental problems during a site investigation.For example, discolored soils or rock fragments from prior spills, or contaminated groundwater may bedetected. The geotechnical engineer and the field supervisor should attempt to identify possible contaminationsources prior to initiating fieldwork. Based on this evaluation, a decision should be made whether a sitesafety plan should be prepared. Environmental problems can adversely affect investigation schedules andcost, and may require the obtaining of permits from State or Federal agencies prior to drilling or sampling.
At geotechnical exploration sites where unknown or unexpected contamination is found during the fieldwork,the following steps should be taken:
1. The field supervisor should immediately stop drilling and notify the geotechnical engineer. The fieldsupervisor should identify the evidence of contamination, the depth of contamination, and the estimateddepth to the water table (if known). If liquid-phase product is encountered (at or above the water table),the boring should be abandoned immediately and sealed with hydrated bentonite chips or grout.
2. The project manager should advise the environmental officer of the governing agency and decide if specialhealth and safety protocol should be implemented. Initial actions may require demobilization from thesite.
3.5 COMMON SENSE DRILLING
Drillers performance is commonly judged by the quantity of production rather than the quality of the boringsand samples. Not surprisingly, similar problems develop throughout the country. All geotechnical engineersand field supervisors need to be trained to recognize these problems, and to assure that field information andsamples are properly obtained. The following is a partial listing of common errors:
C Not properly cleaning slough and cuttings from the bottom of the bore hole. The driller should not samplethrough slough, but should re-enter the boring and remove the slough before proceeding.
C In cohesionless soils, jetting should not be used to advance a split barrel sampler to the bottom of theboring.
C Poor sample recovery due to use of improper sampling equipment or procedures.
C When sampling soft or non-cohesive soils with thin wall tube samplers (i.e., Shelby tube) it may not bepossible to recover an undisturbed sample because the sample will not stay in the barrel. The driller shouldbe clearly instructed not to force recovery by overdriving the sampling barrel to grab a sample.
C Improper sample types or insufficient quantity of samples. The driller should be given clear instructionsregarding the sample frequency and types of samples required. The field supervisor must keep track ofthe depth of the borings at all stages of the exploration to confirm proper sampling of the soil and/or rockformations.
C Improper hole stabilization. Rotary wash borings and hollow-stem auger borings below the groundwaterlevel require a head of water to be maintained at the top of the casing/augers at all times. When the drillrods are withdrawn or as the hollow stem auger is advanced, this water level will tend to drop, and mustbe maintained by the addition of more drilling fluid. Without this precaution, the sides of the boring maycollapse or the bottom of the boring may heave.
3 - 36
C Sampler rods lowered into the boring with pipe wrenches rather than hoisting plug. The rods may beinclined and the sampler can hit the boring walls, filling the sampler with debris.
C Improper procedures while performing Standard Penetration Tests. The field supervisor and driller mustassure that the proper weight and hammer drop are being used, and that friction at the cathead and alongany hammer guides is minimized.
Figure 3-25. Views of Rotary Drill Rigs Mounted on Trucks for Soil & Rock Exploration.
